Apple unveiled iOS 8 on Monday at its Worldwide Developers Conference, but the company only scratched the surface of the hundreds of changes coming to its iPhone and iPad operating system.

There were several highlights at Apple's keynote event, but there are even more hidden gems that the tech giant didn't get to fully explain:

Hey Siri: Apple users will be able to summon their voice assistant in iOS 8 by simply saying "Hey Siri" -- no need to press any buttons. It's a direct reaction to Google, whose voice assistant can be activated on some Android devices with the words "OK Google Now."

Siri Shazam: By asking, "Hey Siri, what song is this?" it will recognize what songs are playing on home radio, in restaurants and elsewhere, using technology developed by Shazam.

Adjust message storage: In settings, user can adjust how long messages are stored, choosing from forever, one year or 30 days. The system will automatically delete non-eternal messages accordingly, freeing up storage space.